Evolution of Sexual Reproduction - Two-fold Cost of Sex

Two-fold Cost of Sex

In most multicellular sexual species, the population consists of two sexes, only one of which is capable of bearing young (with the exception of simultaneous hermaphrodites). In an asexual species, each member of the population is capable of bearing young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation. The cost was first described in mathematical terms by John Maynard Smith. He imagined an asexual mutant arising in a sexual population, half of which comprises males that cannot themselves produce offspring. With female-only offspring, the asexual lineage doubles its representation in the population each generation, all else being equal. Technically this is not a problem of sex but a problem of some multicellular sexually reproducing organisms. There are numerous isogamous species which are sexual and do not have the problem of producing individuals which cannot directly replicate themselves. The principal costs of sex is that males and females must search for each other in order to mate, and sexual selection often favours traits that reduce the survival of individuals.

Evidence that the cost is not insurmountable comes from George C. Williams, who noted the existence of species which are capable of both asexual and sexual reproduction. These species time their sexual reproduction with periods of environmental uncertainty, and reproduce asexually when conditions are more favourable. The important point is that these species are observed to reproduce sexually when they could choose not to, implying that there is a selective advantage to sexual reproduction.

It is widely believed that a disadvantage of sexual reproduction is that a sexually reproducing organism will only be able to pass on 50% of its genes to each offspring. This is a consequence of the fact that gametes from sexually reproducing species are haploid. This, however, conflates sex and reproduction which are two separate events. The "two-fold cost of sex" may more accurately be described as the cost of anisogamy. Not all sexual organisms are anisogamous. There are numerous species which are sexual and do not have this problem because they do not produce males. Yeast, for example, are isogamous sexual organisms which have two mating types which fuse and recombine their haploid genomes. Both sexes reproduce during the haploid and diploid stages of their life cycle and have a 100% chance of passing their genes into their offspring.

The two-fold cost of sex may be avoided by species in many ways. Females may eat males after mating, males may be much smaller or rarer, or males may help raise offspring.